This invention relates to technique of ADPCM (Adaptive Differential Pulse Code Modulation) for implementating sound synthesizers.
Generally, there are two methods of sound synthesis. One is the Frequency Domain method, including Linear Prediction Code (LPC) method and Formant method, etc., which was mainly developed for human voice but not suitable for the animal sound. Another sound disposition method is the Time Domain method, including Pulse Code Modulation (PCM), Adaptive Differential Pulse Code Modulation (ADPCM) and Continuously Variable Slope Delta Modulation (CVSD).
The advantage of the PCM, ADPCM, CVSD technologies is that their control circuits are quite simple. But the shortcoming is that they need quite large memories. Therefore, how to reduce the needed memory while not sacrificing the quality of sound is a concern of research.
In general, a silence sound exists between two words or two sentences in human voice and it takes 20%-40% of the whole sound. So does the animal sound. Therefore, if the silence sound can be eliminated while saving the silence data and adding it automatically during regeneration, the needed memory for sound storage can be decreased and distortless quality is maintained. The disposition of silence sound in the TMS 5200 Speech Synthesizer produced by TI CO., U.S.A. is encoded by four bit "0" to express 20 ms silence sound; U.S. Pat. No. 4,449,190 develops a set of codes to express silence sound; U.S. Pat. No. 4,701,937 is based on the PCM technology and defines a fixed value automatically at the output for the silence sound. The difference between this invention and the above-mentioned patents is to utilize the circuit structure of the ADPCM technology to make an adder add 0 for silence sound and the power remains at the same level with the previous signal.
The sound effects of animals or others have the property of sound repetition. For example, the bark of a dog "Wang-Wang", "Wang -Wang -Wang" which only needs repetition of once of the first sound twice of the second sound. Hence, half or even more memory is redundant.
As to the ADPCM Coding and Encoding technologies, many dissertations have been published in the past. The basic structure of this invention is based on the theory by S. L. Bates "A Harward Realization of PCM-ADPCM Code Converter" M.S. thesis, MIT, Cambridge, Mass., Jan. 1976. In this thesis, multiplication is replace by addition. The theory and structure of ADPCM are stated very clearly in the thesis and there is no need to describe here. One difference between the present invention and the above-mentioned thesis is that the present invention encodes the data in the computer and stores those codes of sound in the memory. Afterwards, the memory device will be combined with the Decoder to become the Speech Synthesizer. Another difference is that the silence sound is represented by 0000 code which is not stored in the memory but generated by Address Generator Decoder and the stop signal is represented by 1000. When the control circuit detects and gets the code of 1000, it terminates decoding and the sound ends.